The field of the invention is photon emission tomography scanners and in particular, a high density electrical interconnect system suitable for use with the many closely spaced detectors of such scanners.
Positrons are positively charged electrons that are emitted by radionucleotides which have been prepared using a cyclotron or other device. The radionucleotides most often employed in diagnostic imaging are fluorine-18 (18F), carbon-11 (11C), nitrogen 13 (13N), and oxygen 15 (15O). Radionucleotides are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances such as glucose or carbon dioxide. One common use for radiopharmaceuticals is in the medical imaging field.
Radiopharmaceuticals may be used in imaging by injecting the radiopharmaceutical into a patient where it accumulates in an organ of interest. It is known that certain specific radiopharmaceuticals become concentrated within or are excluded from certain organs. As the radiopharmaceutical becomes concentrated within the organ of interest, and as the radionucleotides decays and emits positrons, the positrons travel a very short distance before they encounter an electron upon which the positron is annihilated and converted into two photons or gamma rays.
This annihilation event is characterized by two features which are pertinent to medical imaging and particularly to medical imaging using photon emission tomography (PET). First, each gamma ray has an energy of essentially 511 keV upon annihilation. Second, the two gamma rays are directed in substantially opposite directions. If the general location of the annihilation can be identified in three dimensions, the shape of the organ of interest can be reconstructed for observation.
To detect annihilation locations, the PET scanner includes a plurality of detector units each connected to a detector module communicating with a central processor having coincidence detection circuitry. An example detector unit may include an array of crystals (e.g., 36) and a plurality of photo multiplier tubes (PMTs). The crystal array is located adjacent to the PMT detecting surface. When a photon strikes a crystal, the crystal generates light which is detected by the PMTs. At the detector modules, the signal intensities from the PMTs are combined and compared to a threshold (e.g., 100 keV). When the combined signal is above the threshold, an event detection pulse (EDP) is generated and communicated from the detector module to the processor.
The processor identifies simultaneous EDP pairs which correspond to crystals which are generally on opposite sides of the imaging area. Thus, a simultaneous pulse pair indicates that an annihilation has occurred on a straight line between an associated pair of crystals. Over an acquisition period of a few minutes, millions of annihilations are recorded, each annihilation associated with a unique crystal pair. After an acquisition period, recorded annihilation data is used by any of several different well-known procedures to construct a three-dimensional image of the organ of interest.
The determination of the coincidence by the processor, and thus the ability to generate an image, requires that the EDP signals be communicated with minimal distortion from the detector modules to the processor. This is necessary so that the time and energy level of the EDPs may be accurately determined. This in turn requires that the interconnections between the detector modules and the processor have a well-defined impedance, low signal cross-talk and low signal attenuation. These characteristics may be met by coaxial cable. Unfortunately, the large number of signals that must be communicated in a PET scanner from multiple detector units to the processor, makes the use of standard coaxial cable prohibitively expensive and impractically bulky.
Near coaxial cable performance can be obtained from a type of specially configured shielded ribbon cable in which many parallel conductors are joined together in a ribbon by a common insulating material. The ribbon is then covered by a conductive foil shield. By connecting the foil shield and every other conductor within the ribbon cable to a return potential, the signal carrying conductors are effectively surrounded by separate shields, much like the shielding of a coaxial cable. The balancing of the signals and current return reduces the emissions of the cable and the ribbon configuration allows convenient, high-density termination of the cable using multi-pin connectors and the like. Shielded ribbon cables of this type are commercially available from the 3M Company of Minnesota under the name “low skew pleated foil cable” (PFC).
This pleated foil cable, while providing the necessary controlled transmission characteristics, is substantially more susceptible to external electromagnetic interference and thus has proven unsuitable for use in PET scanners. While the inventors do not wish to be bound by a particular theory, this susceptibility problem may be because flat ribbon cable presents a larger open loop area, especially in less than ideal grounding configurations.